Based off of articles like this https://www.makeuseof.com/tag/linux-smartphone-operating-systems/ the best one, to me, looks like Plasma Mobile (which requires a Nexus 5).
Is anyone using a Linux-based mobile operating system? Do you have any advice?
The operating system was pretty solid, and I really liked the lock screen design and pull-out Unity sidebar. It was fast and sleek, but didn't quite yet have the polish of something like iOS - which of course is to be expected for something early in development.
As usual with less well-known mobile operating systems, there were very few apps available. You could get many of the hacker-y/privacy ones that us HN lot probably use, but if you wanted a 'mainstream' social app then you probably couldn't.
After a few months I did end up flashing Android onto the phone, mainly because of the early Ubuntu Touch versions lacking features.
Hardware wise, I still have the handset today and it's great - performance wise the phone was ahead of its time when it was released in 2014, and it still performs well.
Also, did Ubuntu come as the default OS, or was it an Android phone onto which you installed Ubuntu?
It came with Ubuntu pre-installed. I believe that the pre-installed Ubuntu ones have an unlocked bootloader, so you can put whatever you want on it as long as it's compatible. I know that a few people got CyanogenMod working.
I currently have Android Flyme on it, which is the standard Meizu OS that comes with the normal version of the phone. It's really good, but I would say lacking in security. I use it as a 'dirty' phone, for connecting to public Wi-Fi and insecure stuff like that, so I don't have anything confidential or important on there, and I never sign-in to things in the browser or apps.